There has been a long search for radially-repelling ion confinement systems with axially superimposed DC electric fields for various types of applications: for ion guides, for the generation of monoenergetic ion beams, and in particular for collision cells used to fragment and thermalize ions. In such systems it is possible, for example, to not only fragment ions by means of collisions but also to thermalize them, the ions being transported to the ion exit at the end of the system either subsequently or simultaneously by a weak axial DC field. Even for high-resolution mass filters with two-dimensional quadrupole RF fields, a DC potential profile along the axis would offer completely new possibilities, particularly with respect to high transmission and operation at a high damping gas pressure. The term “two-dimensional quadrupole fields” is used to describe the fields which appear in systems comprising four round or hyperbolic lengthy electrodes, as is the usual practice in specialist literature.
Since there are numerous applications for the quadrupole RF electrode systems with their radial retaining force, and hence numerous ways of denoting them, for example mass filters, ion guides, fragmentation cells or thermalization cells, the term “quadrupole systems” is used below where a more precise specialization is not required. What is meant by these quadrupole systems, figuratively speaking, is the confinement of ions in a virtual tube with radially increasing repelling forces. Quadrupole systems with axial potential gradients correspond to sloping tubes in which the content flows in one direction under the influence of the slope.
The simplest (and longest known) solution for the superimposition of a longitudinal electric field consists in making a quadrupole electrode system out of four thin resistance wires, along each of which a DC voltage drop is generated. But the thin wires require a quite high RF voltage in order to generate the quadrupole RF field, since the largest voltage drop occurs in the immediate vicinity of the thin wire. Furthermore, the resistance must not be too high, otherwise the RF voltage fed at the ends cannot propagate along the wires sufficiently quickly. It is therefore only possible to generate rather small DC voltage drops along the wire. Also, it is difficult to generate a desired profile of the DC electric field along the axis. Moreover, the pseudopotential barrier between the wires is very low; the ions can escape very easily.
Pseudo-hyberbolic quadrupole systems comprising a large number of clamped wires which imitate the four hyperbolic surfaces of an ideal quadrupole system represent a further possibility. Hyperbolic quadrupole systems replicated in wire like this were already being used around 40 years ago by Wolfgang Paul and his coworkers (Nobel-price winner Wolfgang Paul is the inventor of all quadrupole systems). These quadrupole systems made from wires are difficult to produce, however, and not very precise, but they do provide a simple way of generating an axial DC field by generating voltage drops along the wires.
Other ion storage systems which have an electrically generated forward drive are known from U.S. Pat. No. 5,572,035 (J. Franzen). This patent concerns different types of ion guides, for example a system comprising only two helical, coiled conductors in the shape of a double helix, operated by being connected to the two phases of an RF voltage. Another guide system consists of coaxial rings to which the phases of an RF AC voltage are alternately connected. Both systems can be operated in such a manner that an axial feed of the ions is generated. It is thus possible to make the double helix out of resistance wire across which a DC voltage drop is generated. The individual rings of the ring system can be supplied with a DC potential which decreases in steps ring by ring, as described in the patent.
U.S. Pat. No. 5,847,386 (B. A. Thomson and C. L. Jolliffe) describes seven different ways to generate an axial voltage drop in quadrupole round-rod systems. Since five of these types distort the inner quadrupole potential, we here consider only the two types which leave the RF quadrupole potential undisturbed: (a) a quadrupole rod system made of nonconducting round rods to which resistance layers have been applied, a voltage drop being generated along each of these; and (b) a quadrupole rod system whose rods are made of nonconducting thin-walled ceramic tubes, coated on the outside with a high-resistance layer for a DC voltage drop and on the inside with a metal layer for the RF supply; the RF voltage being intended to act through both the insulator and, with slight attenuation, through the high-resistance layer as well, in order to form the quadrupole RF field.
These devices are, however, not particularly satisfactory: System (a), comprising nonconducting rods with resistive coating, conducts the RF voltage only in a limited way (similar to the system made of four resistance wires), so that the RF voltage varies along the system, an occurrence which is extraordinarily damaging for some applications; or the resistive coating must have an extremely low resistance.
System (b), made of thin ceramic tubes (according to the specification, tube walls some 0.5 to 1 millimeter thick) with inner metal coating to generate the RF fields and outer high-resistance layer for the DC voltage drop, is also very disadvantageous. The aim of the inventor as given in the specification is that the RF voltage acts through the dielectric ceramic and through the high-resistance layer which, according to the description, should have a resistance of 1 to 10 Megohms per square surface. The specification indicates a penetration of the high-resistance layer by means of the known effect of a “leaky dielectricum” as the following citation describes: “The surface resistivity of the exterior resistive surface 176 will normally be between 1.0 and 10 Mohm per square. A DC voltage difference indicated by V1 and V2 is connected to the resistive surface 176 by the two metal bands 174, while the RF from power supply 48 (FIG. 1) is connected with the interior conductive metal surface. The high resistivity of the outer surface 176 restricts the electrons in the outer surface from responding to the RF (which is at a frequency of about 1.0 MHz), and therefore the RF is able to pass through the resistive surface with little attenuation. At the same time voltage source V1 establishes a DC gradient along the length of the rod . . . ”. (underlining added). A cylinder made of high-resistance material penetrated by RF as a “leaky dielectricum” in precisely this sense has long been known (P. H. Dawson, “Performance of the Quadrupole Mass Filter with Separated RF and DC Fringing Fields”, Int. J. Mass Spectrom. Ion Phys., 25 (1977) 375–392). According to this idea of the penetration of the high-resistance layer (see FIG. 28A of this patent specification and the text cited above) this layer is connected only to the DC voltage source without any contact of its own to the RF source. This invention is not successful in practice: It is not only the fact that the authors underestimate the strength of the RF attenuation when penetrating the high-resistance layer, but also that high dielectric losses occur in the material of the ceramic tubes as a result of the RF, so that the system in the vacuum becomes hot within a short time and can even begin to glow. In addition, the round rods made of the thin ceramic tubes are mechanically not particularly stable. This technology seems to us to be quite unusable; as far as we know it has never been used in practice.
It is remarkable that for quadrupole systems, and particularly for collision cells as well, RF rod systems with round rods are used as a rule, even though hyperbolic systems were introduced 30 years ago for high-quality quadrupole mass spectrometers, said systems providing significantly better separation efficiencies and transmissions. Inexpensive round-rod systems were always considered good enough for the collision chambers, expensive hyperbolic systems were not used at all.
However, from the work of F. von Busch and W. Paul, Z. Phys. 164, 588 (1961) it is already known that in round-rod quadrupole filters there are non-linear resonances which lead to the ejection of certain ions with motion parameters within the Mathieu stability zone which should therefore be stably collected. In three-dimensional RF ion traps, these resonances lead to the phenomenon of the so-called “black holes”, which occur for the same reason in rod systems, particularly in round-rod systems. Round-rod systems contain octopole and higher even-numbered multipole fields of considerable strength superimposed on the quadrupole field, leading to a distortion of the ion oscillations in the radial direction and hence to the formation of higher harmonics of the ion oscillation. Their matching with the Mathieu side bands leads to these resonances. The resonances occur, however, only when the ions undergo relatively wide radial oscillations. For ions lying damped in the axis of the system, the resonances are not effective since there, the higher multipole fields and hence the overtones (higher harmonics) disappear.
In quadrupole systems used as collision cells, the ions are injected with high energy of between 30 and 100 electron-volts. Necessarily large numbers of ions are brought, by means of collision cascades, far outside the central axis. These ions are therefore inevitably subjected to the phenomenon of non-linear resonances if they fulfil one of the numerous resonance conditions. Specific species of daughter ions can thus disappear from the collision cell and hence from the daughter ion spectrum. In the most unfavorable case, even the parent ions selected are subject to this resonance and most of them disappear from the collision cell.
Moreover, round-rod systems have the further disadvantage that the pseudopotential barrier between the rods is quite low (in commercially available systems only some ten to twenty volts) and can easily be overcome by ions with an energy of 50 electron-volts, the minimum usually required for fragmentation processes, by means of a random, laterally deviating collision cascade. This escape affects both parent and daughter ions. The higher the mass of the collision gas molecules, the more ions are lost, because in this case, the angles of deflection per collision are greater. A cascade of a small number of collisions which coincidentally deflect in the same lateral direction is enough to remove the ion from the collision cell. The larger angles of deflection of a small number of collisions are not able to compensate each other statistically as effectively as the large number of smaller angles of deflection in the case of a very light collision gas.
For other quadrupole systems, and even for precision mass filters to some extent, round-rod systems with suitable dimensions have proved to be successful.
In tandem mass spectrometers, the parent ions are generally selected from a primary ion mixture by a quadrupole mass filter; then fragmented in a collision cell. After fragmentation, the daughter ions can be analyzed either by quadrupole mass spectrometers, by time-of-flight mass spectrometers with orthogonal ion injection, by RF ion traps or by ion cyclotron resonance spectrometers. The daughter ion spectrum (or “fragment ion spectrum”) delivers information about the structure of the parent ions. Consequently, at least two types of “quadrupole systems” are used in tandem mass spectrometers: a quadrupole mass filter to select the parent ions, and a quadrupole collision cell to fragment the ion species selected. Usually, there is even an additional thermalization quadrupole for the ions injected into the mass filter (U.S. Pat. No. 4,963,736, D. J. Douglas and J. B. French), and in so-called “Triple Quads” there is a second quadrupole mass filter to analyze the daughter ions, so that this type of system can comprise a total of four quadrupole systems. For some of these quadrupole systems, for example for the thermalization systems, it is highly advantageous to have a forward drive of the ions and, as a rule, this forward drive of the ions must also be switchable and adjustable.
For many quadrupole system applications it is consequently very interesting to generate a potential profile along the axis and to be able to change it while in operation, and also to be able to generate various profiles of the potential characteristic.